1. Field of the Invention
The instant invention relates to Integrated circuit capable of operating at different supply voltages.
2. Discussion of the Related Art
The trend in chip design is towards shrinking geometries. As part of this trend the gate length of a MOSFET, which is the distance between the source and the drain regions underneath the gate region, is also reducing. These shrinking digital CMOS gate lengths, necessitate corresponding reduction in the operating voltages. This change calls for lowering of board supply voltages in order to prevent punchthrough, breakdown and hot-carrier related problems. However, board supply voltages cannot be reduced easily owing to interfaces to external devices.
An on-chip voltage regulator is often the solution to incompatibility with board voltage. In this approach a regulator is used to step down the external board supply to the required chip operating voltage. The on-chip voltage regulators have a dropout voltage equal to the difference between the external board supply and chip operating voltage. This helps address the board supply compatibility issue for the core operating voltages. When the lower board voltages are made available in newer board design the regulator is operated in bypass mode, wherein all the current is passed through a fully-on NMOS/PMOS. This approach has an inherent problem of voltage drop across the switch, which may be unacceptable in high current consuming cores operating at low voltage levels around 1V. To avoid this, a new chip without the regulator is required to be fabricated which is very costly.
FIG. 1 illustrates the ballast transistor (11) implementation in one I/O pad (10). Vg (14) is the gate control voltage coming from a control block. One terminal (15) of the PMOS ballast (11) is connected to the 1.8V pad (12) and other terminal to the 1.2V pad (13).